In a display apparatus such as an organic electro luminescence display apparatus, a liquid crystal display apparatus or the like, a thin film transistor (TFT) is used in a pixel circuit. Examples of thin-film transistors include an a-Si thin film transistor using amorphous silicon (a-Si) for a semiconductor, a low temperature polysilicon (LTPS) thin film transistor obtained by crystallizing a-Si by laser annealing, and an oxide thin film transistor using an oxide semiconductor film.
The a-Si thin film transistor is a thin film transistor containing a-Si in an active layer. The LTPS thin film transistor is a thin film transistor containing LTPS in an active layer. The oxide thin film transistor is a thin film transistor containing an oxide semiconductor in an active layer.
Here, comparing between the characteristics of the LTPS thin film transistor and the oxide thin film transistor, the following differences are observed. The LTPS thin film transistor requires a long channel length, which increases the occupied area. In comparison, the oxide thin film transistor may have a short channel length, which makes the occupied area small. The LTPS thin film transistor generates hysteresis in the source-drain current for the gate voltage. In comparison, the oxide thin film transistor generates minor hysteresis. The LTPS thin film transistor has a high switching performance. In comparison, the oxide thin film transistor has a low switching performance.
In recent years, a semiconductor device has been proposed in which both the LTPS thin film transistor and the oxide thin film transistor are mounted thereto while taking advantage of the characteristics of the two thin film transistors. For example, United States Unexamined Patent Application Publication No. 2015/0055051 proposes a semiconductor device in which an LTPS thin film transistor and an oxide thin film transistor are mounted in a pixel circuit, and a display apparatus comprising the semiconductor device.
However, in the related art such as United States Unexamined Patent Application Publication No. 2015/0055051, a gate insulating layer is formed of one same layer for the oxide thin film transistor and the LTPS thin film transistor. This causes the following problems.
In the manufacturing steps of the LTPS thin film transistor, typically, an activation step and a hydrogenation step are performed after the gate is formed. In the hydrogenation step, an oxide-semiconductor layer of an oxide thin film transistor is exposed to a hydrogen atmosphere. This results in a lowered resistance value for the oxide-semiconductor layer. Accordingly, the formed oxide thin film transistor indicates a normally-on characteristic. Normally-on means that the drain current flows and it keeps turn on state of the transistor when there is no voltage applied to the gate of the transistor. In the case where the oxide thin film transistor is used for a purpose where normally-on is not preferable, a device indicating the normally-on characteristic would be a defective product. This may lead to reducing in yield of semiconductor devices.